1. Field of the Invention
The present invention relates to a method for forming a porous silica film. Porous silica film is useful as a semiconductor insulating film due to the excellent property of low relative dielectric constant (low-k), or as a low refractive index optical film in the display field due to the excellent property of low refractive index (low-N).
2. Description of the Related Art
A porous silica film obtained by thermal-treating solution of a mixture of a hydrolyzable alkoxysilane compound, a water-alcohol type solvent and a pore forming material on a substrate is promising as a semiconductor material for an interlayer insulating film of copper wiring of LSI etc., or a display material which can bring highly efficient takeout of internal light emitting upon lamination on a transparent electrically conducting film. Further, porous silica film can be applied to novel utility such as anti-reflection to incident light from the outside in a solar cell.
Such attention to porous silica film is paid due to the property of low specific dielectric constant value below 2.5 and the low refractive index property in addition to simplicity of a wet process through a sol-gel method (see, for example, Non-Patent References 1 and 2).
However, on the other hand, some concerns are caused by deficiency in the mechanical strength because of the porous structure. For example, in a semiconductor process, when chemical mechanical polishing (CMP) is performed for forming a copper dual damascene wiring structure, or when high speed wire bonding is performed with a bonding apparatus, porous film is destructed due to deficiency in the structural strength in some cases. Such a deficiency in the strength and the possibility of deterioration in the insulating performance due to adsorption of a moisture into a pore are the reasons why porous film is prevented from being in use. In addition, also in the display field, for the improvement against deficiency in the strength of a porous film, a reinforcing layer for reinforcing the strength is inserted as an intermediate layer to a laminated structure in some cases. Since such improvement is contrary to demand for simplification in a multi-layered structure, deficiency in the strength of porous silica film, as well as developing for reduction of film thickness and surface flattening become an important problem.
[Non-Patent Reference 1]
H. Hanahata, S. Matsuno, M. Miyamoto, T. Ioka and T. Tanabe, “Proc. Advanced Metallization Conf.”, 2000, p. 103
[Non-Patent Reference 2]
K. Suzuki, Y. Nakata, I. Sugiura, T. Owada, S. Fukuyama and E. Yano, “Proc. Advanced Metallization Conf.”, 2001, p. 4
In view of the aforementioned problems, an object of the present invention is to provide a method for forming a porous silica film having such a mechanical strength that overcomes the deficiency in the strength which has ever been pointed out. The strength sought in this case is presumed to be consistent with a practical level, for example, in the semiconductor field and the display field.